The increasing volume of traffic, and particularly heavy traffic, has created a severe problem on many roads and streets in this country. This problem has resulted from elastic type failures in pavements which cause a "chicken-wire," or "alligator" cracking pattern in the pavement surface. This cracking is caused by fatigue of the pavement surface from repeated deflection. Conventional repairs by asphalt overlays are usually effective for a short period only and many other more drastic major repairs such as replacing the pavement surface or the pavement surface and its foundation, are too expensive and often as ineffective as asphalt overlays.
The so-called "flexible-type pavement" is actually not a particularly flexible structure. There are occasions when flexible-type pavements could be classified as very brittle, particularly in cold weather or when the pavement surface has suffered a long period of embrittlement from oxidation and age. The cracking caused by this lack of flexibility has created a tremendous problem, when considered on a nation wide scale. Traveling over the streets and highways of this country, one can seldom go more than a few miles without finding distressed pavement which is basically caused by repeated flexing by the surface of the pavement under the traffic loads.
This type of failure has been variously defined as flexure cracking, elastic-type failure, and fatigue failure. It is characterized by multiple cracking of the "chicken-wire," or "alligator" type pattern without plastic deformation of the pavement surface. The cracking is due to fatigue of the bituminous pavement mixture from repeated deflection of the pavement surface under vehicle load and subsequent recovery of the pavement surface. This deflection and recovery is caused by elasticity of some member of the substructure or foundation of the pavement surface. Fatigue failure is the most prevalent of the three most common types of failure occuring in flexible-type pavements. The other types of failure are:
1. The plastic type of failure, which is manifested by cracking in the pavement surface of the same character as found in elastic-type of failure, but is also accompanied by plastic deformation of the pavement surface. The surface is depressed under the loaded area and usually slightly raised at one or both sides of the loaded area. This type of failure is usually caused by an inadequate thickness of base material and is no longer a serious problem on highways or streets built under modern design criteria; and PA1 2. The surface-type failure, which is characterized by attrition, or stripping and emulsification of the asphalt in a surface of the pavement. There is raveling and loss of material in the surface but no significant amount of cracking in the surface. Although this type of failure is very common, it is not as serious as fatigue-failure because it can be corrected by the application of a seal coat.
Fatigue cracking resulting from elastic-type failure is entirely different from the above two types of failure, and solutions to fatigue cracking have not only been difficult and expensive, but in many cases quite uncertain in their result because there is resilience in some member of the substructure. This resilience must be counteracted by either making the substructure or the surface so rigid that it cannot bend, or by making the surface so flexible that it will take the bending. Part of the difficulty in solving this problem lies in the fact that the deflections required to produce elastic-type failure are so small that almost complete elimination of the resilience in the substructure is required. Repeated deflections of a very small order are sufficient to produce this type of failure. Various authorities have given figures for a critical deflection which range from 0.010 to 0.050 inches with a certain probability that the critical deflection would vary considerably for pavements of different thicknesses, composition, asphalt grade, asphalt content, asphalt quality, prevailing temperatures, and a radius of the deflection curve (see McDonald, C. H.; The Elastic Type of Pavement Failure and Some of Its Causes; 38th annual Conf. of WASHO; (2) Hveem, F. N.; Pavement Deflections and Fatigue Failures; HRB BULL. 114 pp. 43-79, 1955; (3) McDonald, Charles H.; The Flexural Failure of Sand-Asphalt Mixes as Related to Resilient Subgrades, Highway Materials Conf., Denver, CO, 1959.).
Complicating the solution to the problem of repairing fatigue cracking from elastic-type failure is the fact that the source of such a small magnitude of elasticity may be difficult to determine. The elasticity may be either in the subgrade, subbase, or base course. An increase in the normal moisture content of even a good subgrade, caused by frost action for instance, may cause the subgrade to become "quickie" resulting in a condition where vehicle load is born by hydrostatic pore pressure. Although such a condition does not ordinarily last for a long time, there is almost no reasonable thickness of overlying material or pavement that will prevent the deflection caused by the vehicle load on the pavement surface. The pavement surface of a four foot fill over a quickie soil has been observed to visibly deflect under vehicle load. This condition also develops in densely graded base courses through frost action.
Certain materials present in soils, such as mica, have elasticity within themselves, and the economic necessity of using local materials may require that these materials be incorporated in the structure. Such materials are often the only ones available in the particular area that can be used without incurring excessive costs for preparation and construction of the substructure. Perhaps the most common cause of substructure elasticity is entrapment of minute quantities of air in fine-grained subgrade soil. Any soil which is capable of moderate capillary pressure can entrap air under certain moisture conditions by holding it in pores which are sealed on all sides by capillary moisture. Capillary pressure is sufficient to prevent the air from being expelled under traffic loading. If enough of these entrapped air cells are involved in a substructure, the structure has a pneumatic character. In extreme cases such soils have an almost rubber-like elasticity when pressed between the fingers. The moisture content need only be slightly above optimum to entrap air. This type of soil is surprisingly prevalent throughout the United States. In my opinion, the increasing use of cement-treated bases is, whether recognized or not, an attempt to overcome this problem of substructure elasticity by stiffening the substructure with cement. The so-called "up-side-down" method of construction in which the subbase is cement-treated, rather than the base, is quite obviously an attempt to stiffen the substructure against resilience from an underlying member. This is practiced rather commonly in New Mexico and Arizona (see Johnson, Charles W.; "Comparative Studies of Various Combinations of Treated and Untreated Bases and Subbases for Flexible Pavements", ARB BULL. 289, p pp. 44-56, 1961; and Arizona Highway Department, Special Provisions, Interstate Projects on I-10-4, "Tucson to Picacho Peak.").
The use of rigid portland cement concrete pavements has also been quite effective; however, the cost is generally prohibitive for indiscriminate use. Again, the obvious motive in using rigid concrete pavements is to make the pavement structure so rigid that it will not be affected by resilience of the substructure.
An attack against this type of failure, elastic-type failure, has also been mounted from the other standpoint of attempting to make the bituminous mixture more flexible (see McDonald, Charles H.; The Need for Greater Flexibility in the Surface of Flexible Type Pavements, Conf. on Soils Eng., Univ. of Ariz. Tucson, 1954). This has been done by the use of open-graded plant mixes employing very heavy asphalt films on each aggregate particle of the pavement mixture. These mixes have large void spaces so that the high asphalt content, in relation to surface area, will not cause distress. This type of pavement design has helped to ameliorate the situation, but it has not been a cure all.
Similarly, small percentages of rubber incorporated in mixes have also been used. These small percentages of rubber have undoubtedly been beneficial, although information on the degree of success obtained with these mixes for this purpose appears to be somewhat limited. It is my opinion, that the cost of these materials has prevented the use of rubber in the amounts necessary to give the pavement true elasticity. I recognized that an entirely new approach was needed to repair pavements subject to elastic-type failure and that the approach I developed and invented, which is described below, is completely different in its use of rubber from anything which I have read. My approach embodies the use of a relatively high percentage of rubber, combined with asphalt, in a relatively thin application to the pavement surface. The purpose is to keep the overall cost in balance but still obtain maximum elasticity of the patching material. This approach is unique and, to this date has been completely successful in some extremely difficult situations.
Asphalt-rubber compositions are described and claimed in many patents; however, none of these patents disclose the unique elastomeric material that I have prepared from rubber and asphalt. Preparations of asphalt containing rubber have been prepared in the past by workers in the art. For example, the Wilkinson U.S. Pat. No. 108,666 discloses a roofing compound composed of ground anthracite coal, ground gypsum, ground tan-bark, India rubber dissolved and prepared coal tar and/or commercial pitch. In the Tickstone U.S. Pat. No. 1,590,644 a hard composition containing rubber and bitumen is disclosed which is useful as a substitute for porcelain, earthware, ebonite, vulcanite and the like is disclosed. This composition contains principally slate powder and lesser amounts of ground rubber and optionally bitumen and/or coloring matter. The Sadtler U.S. Pat. No. 1,758,913 discloses a rubberized-asphalt mixture which is useful as a road covering. The mixture is prepared by adding aggregate to a pug mill; adding liquefier or asphalt-solvent to saturate the entire aggregate; adding rubber to the saturated aggregate so the finished mixture contains only one-half of one percent rubber based on the weight of the total asphalt added; adding asphalt or other bituminous material to the mixture at a temperature of 250.degree. F. or higher. The Grant U.S. Pat. No. 2,040,256 discloses a rubberized-asphalt composition for sealing pipe joints and the like. The composition is prepared by melting asphalt at a temperature not in excess of 180.degree. C. (375.degree. F.). Ground tacky rubber is added to the molten asphalt. The resulting mixture is raised to a temperature of 245.degree. C. (475.degree. F.) for a period of not less than 10 minutes. The temperature is maintained while the mixture is stirred until no lumps of rubber are detectable in the mixture. The resulting composition consists of 0.5 to 15% rubber and 99.5 to 85% asphalt. The ductility of the composition is slightly less than that of the asphalt and its penetration is not more than 2% less than that of the asphalt. The Rhodes et al. Pat. No. 1,884,240 discloses a rubberized-tar product prepared by heating and stirring rubber, water-gas tar and coal tar and/or pitch until a homogenous mass is obtained. Sulfur is added to the mixture and thoroughly mixed therein. The Taylor U.S. Pat. No. 2,686,169 discloses a method of incorporating rubber latex into hot bitumen, the resulting composition contains 2 to 6% rubber. The Endres et al U.S. Pat. No. 2,700,655 discloses a powdered rubber-containing composition for incorporation in the asphalt. The powdered composition contains from 10 to 50% rubber and from about 90 to 50% filler. Dasher U.S. Pat. No. 2,853,742 discloses a method of producing powdered rubber from scrap vulcanized rubber material which can be employed for mixing with asphalt for the production of bituminous concrete paving mixtures as well as in the production of various types of asphalt coatings and similar compositions in which it is desired that a portion of the rubber be present in the coating. The rubber is prepared in a Banbury machine. The Endres et al U.S. Pat. No. 3,127,367 discloses a method and apparatus for adding latex to hot asphalt to obtain a composition containing between 1 and 2% rubber. The Endres et al U.S. Pat. No. 3,202,623 discloses a dry, powdered rubberized composition for incorporating into asphalt. The composition is prepared by combining a water suspension of hard bitumen with rubber latex and then co-precipitating the mixture by means of a coagulant to yield a product containing 5 to 40% by weight rubber. The Peaker et al U.S. Pat. No. 3,242,114 discloses a method of dispersing a rubber-resin composition into asphalt. The resulting composition contains from 1 to 20 parts of rubber per hundred parts of asphalt. The Endres U.S. Pat. No. 3,253,251 discloses paving blocks composed of rubberized bitumen cement and rubber aggregate. This invention can be visualized as blocks of aggregate rubber particles bound by rubberized-asphalt cement. The rubberized bitumen cement contains a very small percentage of rubber.
A commercial product, Ramflex, a powdered rubber specially devulcanized in less than 5 minutes for use in combination with asphalt and aggregate, for asphaltic-type pavement is produced by the U.S. Rubber Reclaiming Company, Inc. RAMFLEX rubber is mixed in a pug mill with asphalt and aggregate. Five to ten percent of RAMFLEX rubber is used for each part of asphalt employed in the total mixture. The total mixture is prepared by adding hot stone or sand and filler to a pug mill in the usual manner; then RAMFLEX rubber is added to the pug mill and mixed 10 to 20 seconds; finally the asphalt is mixed therein for an additional 30 seconds. The material is then ready for application.
The above patents show that rubberized-asphalt compositions are old in the art. However, not one of the previous workers in the field made the startling discovery that when certain portions of rubber and asphalt are heated and mixed together a jellied composition is formed which makes an excellent elastomeric paving repair composition.